One of the conventional optical rotation encorders for an X-Y direction input device includes an X-direction photoelectric apparatus and a Y-direction photoelectric apparatus. As shown in FIG. 1, each photoelectric apparatus 1 includes a circular code wheel 11 having plural equiangularly distributed apertures, two light-emitting diodes 12, 13 separately mounted beside one side of the circular code wheel 11, and two light responsive elements 14, 15 separately mounted beside the other side of the circular code wheel 11. Therefore, the light emitted from the light-emitting diodes 12, 13 can pass through the apertures to the light responsive elements 14, 15 when the circular wheel rotates, so the light responsive elements 14, 15 can intermittently receive the light, and then a series of pulse signals are generated. When the input device is translated along a surface, a ball of the input device is contacted against the surface and then rolled, so the code wheel whose shaft contacts against the ball is synchronously rotated and the pulse signals corresponding to the angular movement of the ball in the X-direction and Y-direction and further corresponding to the desired cursor movement on a related monitor are obtained. The chief disadvantage of the aforementioned optical rotation encoder for an X-Y direction input device is that its assembly is relatively complicated owing to the relatively large number of the involved components. Besides, the distances between the light-emitting diodes 12, 13 and the light responsive elements 14, 15 should be carefully considered by a precise calculation in order to ensure an accurate operation of the optical rotation encoder. Referring to FIG. 2, there is another conventional optical rotation encoder for an X-Y direction input device whose photoelectric apparatus 2 includes a circular code wheel 21 having plural equiangularly distributed apertures, a planar light source 23 such as a light-emitting diode mounted beside one side of the circular code wheel 21, and a pair of vertically aligned light responsive elements 25 (e.g., a pair of vertically aligned phototransistors). Its basic principle and operation are similar to those of the previously described optical rotation encoder, but this conventional technique applies a pair of vertical aligned light responsive elements 25 instead of two separately located light-emitting diodes. In addition, the number of the involved components is reduced and the assembly and the installation thereof are relatively easy because of the use of the pair of vertically aligned light responsive elements 25. The pair of vertically aligned light responsive elements 25 can generate "00", "01", "10", and "11" pulse signals, e.g., when both of the pair of vertically aligned light responsive elements 25 are shielded by the code wheel 21 and cannot receive the light emitted from the light source 23, the "00" pulse signal is generated; and when both of the pair of vertically aligned light responsive elements 25 are rightly corresponding to the apertures and can receive the light emitted from the light source 23, the "11" pulse signal is generated. As shown in FIG. 3, if the combined width of the pair of vertically aligned light responsive elements 25 is 2 W, the size of the apertures of the code wheel 21 must be slightly larger than 2 W. Owing to the facts that the resulting resolution for the related monitor is directly affected by the size of the apertures and that the size of the pair of vertically aligned light responsive elements 25 cannot be reduced (i.e., a reduced size of the pair of vertically aligned light responsive elements 25 will result in an erroneous operation of the input device), if the resulting resolution is to be increased, the size of the code wheel 21 should be increased to allow an increase in the number of the apertures. However, the increase in the size of the code wheel 21 will correspondingly increase the size of the X-Y direction input device. Thus, this conventional input device cannot both meet the demands of an increased resulting resolution and a reduced size.
It is therefore attempted by the Applicant to deal with the above situation encountered by the prior art.